In photography, a conventional camera flash is used to improve image picture quality in low light situations, by illuminating the scene with a burst of visible light while a picture of the scene is taken. For portable devices, such as handheld dedicated digital cameras and multifunction devices referred to as smart phones, the practical choices for an integrated, flash light source include the use of light emitting diodes (LEDs) and gas discharge lamps. An LED flash can be used to provide continuous illumination, which provides good illumination for capturing a rapid sequence of images, such as a video sequence. A gas discharge flash is typically operated in a pulsed manner to provide a very high intensity light but for a relatively short duration, no longer than the period of time the shutter is allowed to remain open to capture the scene for a single picture or frame. It is some times desirable to provide a less intense flash, e.g. during a redeye reduction process where the main flash is immediately proceeded by one or more reduced intensity flashes.
Illumination by flash is provided during the image-framing period (also referred to as the single shutter cycle for taking a picture). A typical range for such a period is 200-300 milliseconds. Some LED flashes are not capable of providing their highest level of illumination for the entire image framing period, and thus have to be pulsed with, for example, one larger pulse and one smaller pulse during the entire shutter cycle. There may also be thermal reasons for pulsing an LED flash.
In other aspects of photography, it is known that a moving object in the scene, or movement of the camera relative to an object in the scene, causes motion blur. In other words, the object appears blurred in the picture. Shortening the exposure time for taking the picture may reduce such blur, provided the image sensor is sensitive enough to capture a sufficient amount of light from the scene during the shorter exposure time. In another technique known as deblurring, a signal processing operation known as deconvolution can be applied to the picture in order to recover the high frequency or edge details that have been lost in the blurred picture. It has been reported that for an improved deblur operation, rather than leaving the shutter open continuously for the entire exposure duration, the camera shutter is “fluttered”, i.e. opened and closed rapidly during a single exposure period, in accordance with a binary pseudo-random sequence or code. This flutter effectively changes the inherent filtering effect of the exposure time, in a way that better preserves the high frequency spatial or edge details in the picture, such that the subsequent deconvolution (deblurring) operation can be more easily performed to recover the edge information in the picture. This so-called coded exposure photography technique or flutter shutter technique has been suggested as being extendable to strobe lighting flashes. This coded flash sequence has been suggested to provide a greater ability to control motion sensing.